1) Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer or a digital copying machine and, more particularly, to a pixel clock generator that regulates a drive timing of a scanning light source.
2) Description of the Related Art
A general configuration of an image forming apparatus such as a laser printer or a digital copying machine is shown in FIG. 19. A laser beam output from a semiconductor laser unit 1001 scanned by a rotating polygon mirror 1002 to form an optical spot on a photosensitive element 1004 serving as a media to be scanned through a scanner lens 1003, and the photosensitive element 1004 is exposed to form an electrostatic latent image. At this time, a photodetector 1005 detects a scanning beam every line to output a signal. A phase synchronizing circuit 1009 inputs a clock from a clock generating circuit 1008 and generates an image clock (pixel clock) the phase of which is synchronized every line by an output signal from the photodetector 1005. This image clock is supplied to an image forming unit 1006 and also supplied to a laser drive circuit 1007 through the image forming unit 1006. The semiconductor laser unit 1001 controls emission time of a semiconductor laser according to the image data formed by the image forming unit 1006 and an image clock to control an electrostatic latent image on the photosensitive element 1004.
In such a scanning optical system, a fluctuation of a distance from a rotating axis of a polarizing reflective surface of a polarizer such as a polygon mirror generates an uneven scanning speed of an optical spot (scanning beam) which scans a surface to be scanned. The uneven scanning speed causes displacement of dots to be recorded to flicker an image, thereby deteriorating image quality. Therefore, when a high-quality image is required, au uneven scanning speed must be corrected.
In addition, in a multi-beam optical system having a plurality of light sources, when oscillation wavelengths of the light sources are different from each other, an exposure error occurs in an optical system in which aberration chromatica of a scanning lens is not corrected. The aberration chromatica also causes displacement of dots to be recorded. In addition, a scanning width set when a spot scans a medium to be scanned changes depending on the light sources to cause deterioration of image quality. Therefore, the scanning width must be corrected.
A technology for correcting an uneven scanning speed is disclosed in, for example, Japanese Patent Application Laid-Open No. 11-167081 and Japanese Patent Application Laid-Open No. 2001-228415. Such technology includes a method of basically changing a frequency of a pixel clock to control an optical spot position along a scanning line.
The following method is also known. That is, clocks in a period in which a scanning beam passes through two photodetectors arranged both the ends of a photosensitive element are counted to detect a scanning speed, and, depending on the result, a rotating speed of a polygon mirror is controlled. FIG. 20 depicts an example of the conventional method. In FIG. 20, reference numeral 1105 denotes a photosensitive element; 1107 and 1108, photodetectors arranged both the ends of the photosensitive element 1105; 1111, a scanning speed detecting unit that counts clocks between a detection signal from the photodetector 1107 to a detection signal from the photodetector 1108 to detect a scanning speed and to output a correction signal; and 1112, a polygon motor control unit that controls a rotating speed of a drive motor (not shown) of a polygon mirror 1104 depending on the correction signal. Reference numeral 1101 denotes a semiconductor laser; 1102, a collimator lens; 1103, a cylinder lens; 1106, an f-θ lens; 1109, a folding mirror; and 1110, a toroidal lens.
In another method, for example, in the configuration shown in FIG. 20, a time interval between the detection signals from the photodetectors 1107 and 1108 is measured by a high-frequency clock, and on the basis of the count information, the phase of a pixel clock is shifted to control the position of an optical spot. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-36626.
A conventional system (frequency modulation system) which changes a frequency of a pixel clock generally includes a pixel clock control unit having a complex configuration, and the complexity of the configuration increases in proportion to a decrease in frequency modulation width. For this reason, detailed control cannot be easily performed. Even though an optical beam polarized by the same polarizing reflective surface is used, an uneven scanning speed disadvantageously occurs due to rotating jitter of a polarizer or expansion/shrinkage of a scanning lens caused by a change in temperature. A method of controlling a rotating motor of the polarizer is hard to control the rotating motor at high accuracy.
A system that controls the position of a scanning spot on each scanning line by control of a phase shift of a pixel clock as described in Patent Document 3 is free from the problems posed in the two systems described above. The system is generally advantageously used as a system that corrects an uneven scanning speed or fluctuation of scanning widths at high accuracy.